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ECGD 4122 – Foundation Engineering

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1 ECGD 4122 – Foundation Engineering
Faculty of Applied Engineering and Urban Planning Civil Engineering Department 2nd Semester 2008/2009 ECGD 4122 – Foundation Engineering Lecture 2

2 Revision of Soil Mechanics
Soil Composition Soil Classification Groundwater Stress (Total vs. Effective) Settlement Strength

3 Soil: A 3-Phase Material
Air Water Solid

4 The Mineral Skeleton Solid Particles Volume Voids (air or water)

5 Three Phase Diagram Air Water Solid Idealization: Mineral Skeleton

6 Fully Saturated Soils Water Solid Mineral Skeleton Fully Saturated

7 Dry Soils Air Solid Dry Soil Mineral Skeleton

8 Partly Saturated Soils
Partially Saturated Soils Solid Air Water Mineral Skeleton Partly Saturated Soils

9 Three Phase System Air Water Solid Volume Weight Va Wa~0 Vv Vw Ww WT
VT Vs Ws Volume Weight

10 Weight Relationships Weight Components: Weight of Solids = Ws
Weight of Water = Ww Weight of Air ~ 0

11 Volumetric Relationships
Volume Components: Volume of Solids = Vs Volume of Water = Vw Volume of Air = Va Volume of Voids = Va + Vw = Vv

12 Volumetric Relationships
Volume Components: Volume of Solids = Vs Volume of Water = Vw Volume of Air = Va Volume of Voids = Va + Vw = Vv

13 Specific Gravity Unit weight of Water, w
w = 1.0 g/cm3 (strictly accurate at 4° C) w = 62.4 pcf w = 9.81 kN/m3

14 Specific Gravity, Gs Iron 7.86 Aluminum 2.55-2.80 Lead 11.34
Mercury 13.55 Granite 2.69 Marble 2.69 Quartz 2.60 Feldspar

15 Specific Gravity, Gs

16 Example: Volumetric Ratios
Determine void ratio, porosity and degree of saturation of a soil core sample Data: Weight of soil sample = 1013g Vol. of soil sample = 585.0cm3 Specific Gravity, Gs = 2.65 Dry weight of soil = 904.0g

17 Example Air Water Solid Wa~0 134.9cm3 W =1.00 243.9cm3 109.0g
Volumes Weights

18 Example Air Water Solid 134.9cm3 W =1.00 243.9cm3 109.0cm3 585.0cm3
Volumes s =2.65 341.1cm3 109.0cm3 243.9cm3 134.9cm3 W =1.00

19 Soil Unit weight (lb/ft3 or kN/m3)
Bulk (or Total) Unit weight  = WT / VT Dry unit weight d = Ws / VT Buoyant (submerged) unit weight b = - w

20 Typical Unit weights

21 Fine-Grained vs. Coarse-Grained Soils
U.S. Standard Sieve - No. 200 inches 0.074 mm “No. 200” means...

22 Sieve Analysis (Mechanical Analysis)
This procedure is suitable for coarse grained soils e.g. No.10 sieve …. has 10 apertures per linear inch

23 Hydrometer Analysis Also called Sedimentation Analysis Stoke’s Law

24 Grain Size Distribution Curves

25 Soil Plasticity Further classification within fine-grained soils (i.e. soil that passes #200 sieve) is done based on soil plasticity. Albert Atterberg, Swedish Soil Scientist ( )…..series of tests for evaluating soil plasticity Arthur Casagrande adopted these tests for geotechnical engineering purposes

26 Atterberg Limits Shrinkage limit Plastic limit Liquid limit solid
Consistency of fine-grained soil varies in proportion to the water content Shrinkage limit Plastic limit Liquid limit solid semi-solid plastic liquid Plasticity Index (cheese) (pea soup) (pea nut butter) (hard candy)

27 Liquid Limit (LL or wL) Empirical Definition
The moisture content at which a 2 mm-wide groove in a soil pat will close for a distance of 0.5 in when dropped 25 times in a standard brass cup falling 1 cm each time at a rate of 2 drops/sec in a standard liquid limit device

28 Engineering Characterization of Soils
Soil Properties that Control its Engineering Behavior Particle Size coarse-grained fine-grained Particle/Grain Size Distribution Particle Shape Soil Plasticity

29 Clay Morphology Scanning Electron Microscope (SEM)
Shows that clay particles consist of stacks of plate-like layers

30 Soil Consistency Limits
Albert Atterberg ( ) Swedish Soil Scientist ….. Developed series of tests for evaluating consistency limits of soil (1911) Arthur Casagrande ( ) ……Adopted these tests for geotechnical engineering purposes

31 Arthur Casagrande ( ) Joined Karl Terzaghi at MIT in as his graduate student Research project funded by Bureau of Public Roads After completion of Ph.D at MIT Casagrande initiated Geotechnical Engineering Program at Harvard Soil Plasticity and Soil Classification (1932)

32 Casagrande Apparatus

33 Casagrande Apparatus

34 Casagrande Apparatus

35 Liquid Limit Determination

36 Plastic Limit (PL, wP) The moisture content at which a thread of soil just begins to crack and crumble when rolled to a diameter of 1/8 inches

37 Plastic Limit (PL, wP)

38 Plasticity Index ( PI, IP )
PI = LL – PL or IP=wL-wP Note: These are water contents, but the percentage sign is not typically shown.

39 Plasticity Chart

40 USCS Classification Chart

41 USCS Classification Chart

42 Plasticity Chart

43 U = porewater pressure = wZw
Groundwater U = porewater pressure = wZw

44 Stresses in Soil Masses
P X X Area = A  = P/A Soil Unit Assume the soil is fully saturated, all voids are filled with water.

45 ′ =  - u where, ′ = effective stress
From the standpoint of the soil skeleton, the water carries some of the load. This has the effect of lowering the stress level for the soil. Therefore, we may define effective stress = total stress minus pore pressure ′ =  - u where, ′ = effective stress  = total stress u = pore pressure

46 ′ =  - u Effective Stress
The effective stress is the force carried by the soil skeleton divided by the total area of the surface. The effective stress controls certain aspects of soil behavior, notably, compression & strength.

47 ′z =  iHi - u Effective Stress Calculations where,
H = layer thickness sat = saturated unit weight U = pore pressure = w Zw When you encounter a groundwater table, you must use effective stress principles; i.e., subtract the pore pressure from the total stress.

48 Geostatic Stresses

49 Compressibility & Settlement
Settlement requirements often control the design of foundations This chapter provides a general overview of principles involved in settlement analysis The subject will be dealt with in greater detail in Chapter 7.

50 Increase in Vertical Effective Stress
Due to a Placement of a fill Due to an external load

51 }z f′ }z f′ Consolidation z0′ z′ z0′ z0′ z0′ z′ Before After
H z0′ }z f′ z0′ z′ Voids  e Vv = eVs Vv = (e -  e)Vs Voids Solids Solids Before Vs Vs After

52 Before Loading Point, P u0 0

53 Immediately After Loading
Point, P u0+u 0 + 

54 Shortly after Loading No settlement Long after Loading
0 +  Long after Loading Settlement Complete u0+u u0 0 + 

55 Settlement Distortion Settlement (Immediate)
Consolidation (Time Dependent) Secondary Compression Time Settlement

56 Laboratory Consolidation Test

57 Consolidation Test

58 Test Results

59 Consolidation Plot

60 Idealized Data Test Results

61 Compression Index and Recompression Index

62 Compression Ratio and Recompression Ratio

63 Normally and Over-Consolidated Soils
….. Normally consolidated ….. Over consolidated ….. Under consolidated

64 Over-Consolidation Margin & Over-consolidation Ratio

65 Typical Range of OC Margins

66 Compressibility of Sand and Gravels (Table 3.7)

67 Example 3

68 Settlement Predictions N.C. Clays

69 Settlement Predictions O.C. Clays…… Case I

70 Settlement Predictions O.C. Clays…… Case II

71 Example 4

72 Example 4

73 Example 5

74 Example 5

75 Failure due to inadequate strength at shear interface
Slope Failure in Soils Failure due to inadequate strength at shear interface

76 Shear Failure in Soils

77 Shear Failure in Soils

78 Bearing Capacity Failure

79 Transcosna Grain Elevator Canada (Oct. 18, 1913)
West side of foundation sank 24-ft

80 Shear Strength of Soils
Soil derives its shear strength from two sources: Cohesion between particles (stress independent component) Cementation between sand grains Electrostatic attraction between clay particles Frictional resistance between particles (stress dependent component)

81 Shear Strength of Soils; Cohesion
Dry sand with no cementation Dry sand with some cementation Soft clay Stiff clay

82 Shear Strength of Soils; Internal Friction

83 Mohr-Coulomb Failure Criterion
Shear Strength,S  =  C Normal Stress,  = 

84 Shear Strength is controlled by Effective Stress, '
Slope Surface Potential Failure Surface

85 Mohr-Coulomb Failure Criterion

86 Typical  Values

87 Effect of Pore Water on Shear Strength
Pore water pressure Total Stress, versus Effective Stress, Shear Strength in terms of effective stress

88 Apparent Cohesion Moist beach sand has apparent cohesion
Negative pore water pressures

89 Measuring Shear Strength
Laboratory Direct shear test Unconfined compression test Triaxial compression test Field Vane shear test

90 Direct Shear Test ASTM D-3080; AASHTO T 236

91 Direct Shear Test

92 Direct Shear Test

93 Direct Shear Test Device

94 Direct Shear Test Device

95 Direct Shear Test Data Shear stress

96 Direct Shear Test Data Volume change

97 Peak vs. Ultimate Strength

98 Example: Direct Shear Test
Given: A direct shear test conducted on a soil sample yielded the following results: Normal Stress,  (psi) Max. Shear Stress, S (psi) 10.0 6.5 25.0 11.0 40.0 17.5 Required: Determine shear strength parameters of the soil

99 Example 6

100 Drained versus Undrained Conditions ….
Before loading After loading

101 Drained versus Undrained Conditions ….
Before loading After loading



104 Soil Shear Strength under Drained and Undrained Conditions ….
Drained conditions occur when rate at which loads are applied are slow compared to rates at which soil material can drain Sands drain fast; therefore under most loading conditions drained conditions exist in sands Exceptions: pile driving, earthquake loading in fine sands

105 Soil Shear Strength under Drained and Undrained Conditions ….
In clays, drainage does not occur quickly; therefore excess pore water pressure does not dissipate quickly Therefore, in clays the short-term shear strength may correspond to undrained conditions Even in clays, long-term shear strength is estimated assuming drained conditions

106 Shear Strength in terms of Total Stress
Shear Strength in terms of effective stress Shear strength in terms of total stress u at hydrostatic value

107 Long-term Stability Potential Failure Surface Slope Surface

108 Short-term Stability Potential Failure Surface Slope Surface

109 Shear Strength in terms of Total Stress;  = 0 condition
For cohesive soils under saturated conditions,  = 0.

110 Mohr-Coulomb Failure Criterion
Shear Strength,S  = 0 C Normal Stress, 

111 Mohr’s Circles 3=0 1 Direct Shear Uniaxial Compression

112 Mohr’s Circles  1  3=0 1 Uniaxial Compression Max. shear plane
Horiz. plane 1

113 Mohr’s Circles 3=0 1 Uniaxial Compression

114 Unconfined Compression Test ASTM D-2166; AASHTO 208
3 = 0 1 For clay soils Cylindrical specimen No confining stress (i.e. 3 = 0) Axial stress = 1

115 Unconfined Compression Test Data

116 Unconfined Compression Test

117 Example: Unconfined Compression Test
Given: An unconfined compression test conducted on a soil sample yielded the results shown in the table. Required: Determine undrained shear strength, Su of the soil

118 Example: Unconfined Compression Test

119 Example: Unconfined Compression Test
Su= 21.7psi = 3128 psf qu= 43.45psi=6257 psf

120 Triaxial Compression Test
Unconfined compression test is used when  = 0 assumption is valid Triaxial compression is a more generalized version Sample is first compressed isotropically and then sheared by axial loading 1 3

121 Triaxial Compression Test
1 3 Load applied in 2 stages confining pressure, 3 dev. stress,  = 1 - 3

122 Triaxial Compression Test

123 Triaxial Compression Test

124 Triaxial Compression Test for Undisturbed Soils

125 Drainage during Triaxial Compression Test

126 Triaxial Compression Tests
Unconsolidated Undrained (UU-Test); Also called “Undrained” Test Consolidated Undrained Test (CU- Test) Consolidated Drained (CD-Test); Also called “Drained Test”

127 Triaxial Compression Tests ASTM Standards
ASTM D2850: Unconsolidated Undrained Triaxial Test for Cohesive Soils ASTM D4767: Consolidated Undrained Triaxial Test for Cohesive Soils

128 Triaxial Compression Tests AASHTO Standards
AASHTO T-296: Unconsolidated Undrained Triaxial Test for Cohesive Soils AASHTO T-297 : Consolidated Undrained Triaxial Test for Cohesive Soils

129 Consolidated Undrained Triaxial Test for Undisturbed Soils

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